Why Does a Tethered Drone Battery Still Lose Charge During Flight?

Quick answer: Because the battery and tethered airborne module may share the aircraft load. If the module output voltage is slightly lower than the battery path, if aircraft demand temporarily exceeds module capability, or if cable and thermal conditions reduce available tethered power, the battery can supply part of the current and its state of charge will fall.

A common expectation is that once a UAV is connected to continuous ground power, the original battery should remain at exactly the same state of charge. In many real tethered architectures, that is not how the power path behaves.

When the original battery stays onboard as backup, it may be electrically connected to the aircraft bus at the same time as the airborne DC/DC module. The amount of current supplied by each source depends on voltage, internal resistance, control strategy and the instantaneous aircraft load.

Start With the Power-Sharing Architecture

The simplified system can be viewed as two possible energy sources feeding one aircraft bus: the tethered airborne converter and the original battery. If both paths are active, they do not necessarily contribute equal power.

Tethered DC/DC module → UAV bus ← Backup battery

1. Converter Output Voltage Is Lower Than the Battery Path

Current tends to flow from the source presenting the higher effective voltage at the aircraft bus. If the airborne module is regulated slightly below the battery voltage under load, the battery may continue to contribute current.

This does not automatically mean the tethered module is faulty. The correct set point depends on the aircraft architecture, battery limits and the intended power-sharing strategy. Increasing voltage without understanding the battery and BMS can create a different problem, including unwanted charging or reverse-current risk.

2. Aircraft Demand Temporarily Exceeds the Module Output

Hover power is not the maximum power a UAV will ever request. Takeoff, acceleration, wind correction, payload activation and rapid control inputs can create short power peaks. If the airborne module is sized close to average demand, the battery may cover those transient peaks.

• Normal hover may be fully supplied by the tether.
• Short peaks may be partly supplied by the battery.
• Repeated or prolonged peaks can gradually reduce battery state of charge.

3. Tether Voltage Drop Reduces Available Airborne Power

The tether has electrical resistance. Under load, voltage is lost along the cable. In simplified form, voltage drop increases with current and total loop resistance, while conductor loss follows the familiar I²R relationship.

A longer cable, higher current or higher-resistance conductor can therefore reduce the voltage reaching the airborne converter. If the converter approaches its input limit or thermal limit, the battery may be asked to support more of the aircraft load.

4. Ground-Side Input Is Not Delivering the Expected Power

The airborne module can only deliver power that successfully passes through the complete chain. A generator with insufficient capacity, an incorrect AC supply, ground-station derating or another upstream limitation can reduce available airborne power even if the module itself has a higher nominal rating.

5. Temperature and Cooling Reduce Continuous Capability

Power electronics generate heat. High ambient temperature, restricted airflow or installation near another heat source can reduce the continuous power margin. If the converter or ground equipment derates thermally, the battery may contribute more current than it does in cooler conditions.

6. The Battery Management System Influences Current Flow

Smart batteries and BMS-controlled aircraft can behave differently from a simple passive battery pack. The BMS may limit charge current, disconnect under certain conditions or report state of charge according to its own estimation logic. For proprietary aircraft platforms, battery behavior should be evaluated with the actual aircraft rather than assumed from voltage alone.

How Much Battery Drop Is Acceptable?

There is no universal percentage. A small decline during a long mission can be normal in an architecture where the battery supports transients. A rapid or continuous decline, however, may indicate that the tethered system is not covering the sustained load or that the voltage-sharing relationship is incorrect.

The useful question is not only “Did the battery percentage drop?” but “What current was the battery supplying, under what flight condition, and for how long?”

How to Diagnose the Cause

Record the system behavior under stable hover and then compare it with higher-load events. Useful measurements include:

• Aircraft bus voltage
• Airborne module output voltage and current
• Airborne module input voltage from the tether
• Ground-station output voltage and current
• Battery current direction and magnitude, if available
• Battery state of charge over time
• Ambient temperature and module temperature
• Aircraft operating mode and payload state when the drop occurs

Do Not Solve Battery Drop by Simply Raising Voltage

Increasing airborne output voltage can shift more load from the battery to the tethered module, but the set point must remain compatible with the aircraft, battery and BMS. An incorrect voltage can create unwanted charging or backfeeding. Voltage adjustment should therefore follow the documented power-path design rather than trial-and-error.

What a Well-Matched System Should Achieve

• The tethered system covers the sustained aircraft and payload demand.
• Enough short-duration headroom is available for expected transients.
• The battery remains available as backup if that is part of the design.
• Cable voltage drop and ground-source limits are included in the power calculation.
• The converter operates inside its thermal and voltage limits.
• Battery charging or reverse current is controlled according to the intended architecture.

FAQ

Does battery discharge mean the tethered system is undersized?

Not always. A backup battery may intentionally support short transients. The important distinction is whether the battery is only supporting peaks or is continuously supplying a significant share of the sustained load.

Can I set the airborne module voltage higher so the battery never discharges?

Do not increase voltage without confirming the aircraft and battery limits. Too high a set point can create unwanted charging or reverse-current conditions.

Can tether cable length cause the battery to discharge faster?

Yes, indirectly. More cable can increase electrical resistance and voltage drop. If that reduces the power available to the airborne converter, the battery may contribute more current.

Discuss Your Tethered UAV Project

We develop tethered UAV power systems around the aircraft, payload, operating height and available ground power. For a preliminary review, send the aircraft model or electrical specification, continuous and peak power demand, required tether length, mission application and destination country.